LAND- AND WATERWORKS 



two large air-canals separated only by narrow strips of tissue. 

 Thicker outer walls and a distinct cuticle characterise the epi- 

 dermal cells of the land-form (compare Fig. 93, b and e). The 

 vascular strand of the latter has an almost continuous ring of 



FIG. 93. Transverse sections of the stems of the land- (a-c) and water- 

 forms (d-f) of the Starwort (Callitriche stagnalis}. a and d, entire 

 sections; b and e, epidermis ; c and/, vascular strands, la., lacuna ; 

 Xy., xylem- vessels. 



xylem (Fig. 93, c) in contrast to the one or two xylem elements 

 bordering the central canal (la.) in the water-form, whose vascular 

 strand is slightly smaller (Fig. 93, /). Similar differences are 

 encountered between the structure of the stems of the two forms 

 in other aquatics. 



